Cyclin dependent kinases which control progression through cell cycle checkpoints are fully activated by which of the following:

Options

A. binding to cyclins.
B. phosphorylation by Cdk activating protein kinase.
C. binding to cyclin, plus phosphorylation by a Cdk activating protein kinase.
D. phosphorylation by a tyrosine kinase.

Answer

To solve the question regarding the activation of cyclin-dependent kinases (CDKs) that control progression through cell cycle checkpoints, we can break down the solution into the following steps:

### Step-by-Step Solution

1. **Understanding CDKs**: Cyclin-dependent kinases (CDKs) are enzymes that play a crucial role in regulating the cell cycle. They need to be activated to function properly.

2. **Role of Cyclins**: CDKs are activated by the binding of cyclins. Each phase of the cell cycle has specific cyclins that bind to their corresponding CDKs, which is essential for the progression through the cell cycle.

3. **Phosphorylation Requirement**: In addition to cyclin binding, CDKs also require phosphorylation by CDK-activating kinase (CAK) to become fully active. This phosphorylation adds a phosphate group to the CDK, which is necessary for its activity.

4. **Evaluating the Options**:
- **Option 1**: Binding of Cyclin - This is correct as cyclins are necessary for CDK activation.
- **Option 2**: Phosphorylation by CDK activating protein kinase - This is also correct as phosphorylation is required for full activation.
- **Option 3**: Binding to Cyclin plus Phosphorylation by CDK activating protein kinase - This option combines both requirements, making it the most comprehensive answer.
- **Option 4**: Phosphorylation by Tyrosine kinase - This is incorrect as it does not pertain to the activation of CDKs.

5. **Conclusion**: The most accurate answer is **Option 3**, as it encompasses both the binding of cyclin and the phosphorylation by CDK activating protein kinase, which are both essential for the full activation of CDKs.

### Final Answer
**Option 3: Binding to Cyclin plus Phosphorylation by CDK activating protein kinase.**

---